1. Field of the Invention
This invention relates to a transmission system comprising a transmitter with an encoder for deriving a coded signal from a quasi-periodic signal, the transmitter being arranged for transmitting the coded signal to a receiver via a medium, the encoder comprising a pitch detector for deriving pitch information from the quasi-periodic signal.
The invention likewise relates to an encoder, a detector for detecting the period of a quasi-periodic signal and a method of pitch detection.
2. Description of the Prior Art
A pitch detector to be used in a transmission system as defined in the opening paragraph is known from the journal article "Automatic and Reliable Estimation of Glottal Closure Instant and Period" by Y. M. Cheng and D. O. Shaughnessy, IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-23, pp. 418-423, 1976.
Such transmission systems are used, for example, for transmitting speech signals by a transmission medium such as a radio channel, a coaxial cable or a glass fibre. Alternatively, such transmission systems may be used for storing speech signals on a storage medium such as a magnetic tape or disc. Applications are, for example, automatic telephone answer machines and dictating machines.
A speech signal consists of voiceless and voiced components. A voiceless component of a speech signal occurs when some consonants are pronounced and do not show any periodicity. A voiced component of a speech signal occurs when vowels are pronounced and is more or less periodic. Such a signal is also termed quasi-periodic. An important parameter of such a signal is the period, usually called pitch. For various types of speech encoders it is of great importance to calculate accurately the pitch of the voiced components of the speech signal.
A first method of determining the pitch is calculating the autocorrelation function of the quasi-periodic signal, the pitch information being represented by the difference of the delay between two peaks of the autocorrelation function. A problem is then that a single pitch value is calculated over a signal segment that has a given time duration. Any variations of the pitch in the given time duration cannot be measured, but lead only to an (undesired) widening of the peaks of the autocorrelation function.
In the pitch detector known from said journal article, the pitch information is derived from a cross-correlation function between the speech signal and a modelled response of the human speech system to an excitation signal that is caused by the closing of the vocal cords. The properties of the human speech system are described by linear prediction parameters derived from the speech signal. From this cross-correlation function is derived a signal in which peaks occur that indicate the excitation instants. The average value of this signal is subtracted from this signal and clipped, so that a pulse-shaped signal is obtained in which the pulses denote the excitation instants. It appears that pulses may be lost in signals having a non-constant pitch, or secondary pulses may appear as a result of the average value being temporarily too high or too low. This will lead to a reduced reliability of the pitch detection.